The present invention relates to an image forming method and an image forming apparatus.
In an electrophotographic image forming apparatus, light is applied to a uniformly charged surface of an image carrier such as a photoconductive drum or a photoconductive belt in accordance with printing information to thereby form an electrostatic latent image, then the electrostatic latent image is developed with toner particles, and then the developed toner image is transferred to a recording medium such as paper or resin film and fixed using heat, pressure, light or the like.
The most general way of fixing the toner image is the way using heat rolls. However, the fixing using heat rolls has the following problems: Though heat efficiency is high, initial heating (rising) takes several minutes. Further, toner is apt to be put out of position onto the heat rolls and stain the recording paper. Further, since the recording medium is nipped by a pair of heat rolls, when the recording medium is continuous paper such as paper for computer output, wrinkles and breaks, are apt to be produced due to the paper""s meandering.
In the case of an image forming apparatus using radiant energy of flash light intermittently emitted from a flash lamp such as a xenon light source, toner absorbs radiant energy selectively and enables high-speed fixing. Further, in flash fixing, a flash lamp and recording paper are not in contact. This has an advantage that there is no fear of toner""s being put out of position, or wrinkles and breaks being produced due to the recording medium""s meandering. Another advantage is that a toner image is fixed easily even to sized paper.
In the flash-fixing type image forming apparatus, part of flash light can impinge on the photoconductor as leak light, directly or indirectly, that is, having been reflected by a reflecting plate or shielding plate attached to the flash lamp, a carrying belt, paper and/or the like, in accordance with the flash lamp periodically emitting the flash light, intermittently. This can produce stains on a white ground.
When cut sheet paper is used, the flash-lamp side of the carrying belt is exposed between cut sheets. Therefore, for example, if antireflection treatment such as black coating is applied to the carrying belt, it may reduce the leak light from the flash lamp impinging on the photoconductor. However, when continuous paper is used, the carrying belt is not exposed. Therefore, more intensive leak light from the flash lamp may impinge on the photoconductor. In that case, photo fatigue and transfer memory may be produced at those portions of the photoconductor on which the flash light has impinged, so that the capability to be charged may drop.
Here, photo fatigue means that the capability of the photoconductor to be charged drops at its portions that have received intensive light. As shown in FIG. 4, photo fatigue can be evaluated as follows: After electricity is removed from a photoconductor 1 by a discharging lamp 2, the photoconductor 1 is charged by a main charger 3. Then, flash light from a flash lamp 5 is applied to the photoconductor 1 through a slit 4. A decrease in surface potential xcex941 of the surface of the photoconductor 1 caused by the flash lamp 5 being turned on after the main charging (as shown in FIG. 5) is measured with a surface potential sensor 7 to thereby evaluate photo fatigue.
Transfer memory means, as shown in FIG. 6, that electric charge supplied by a transfer charger 6 having a polarity opposite to the polarity of the photoconductor 1 remains until directly before charging by a main charger 3, so that an increase in surface potential caused by the charging by the main charger 3 reduces, that is, the capability to be charged drops. Transfer memory can be evaluated as follows: After electricity is removed from the photoconductor 1 by the discharging lamp 2, the photoconductor 1 is charged by the main charger 3. Then, the photoconductor 1 is subjected to transfer charging by the transfer charger 6 whose polarity is opposite to the polarity of the main charging. Then, a decrease in surface potential xcex942 of the surface of the photoconductor 1 after the main charging (as shown in FIG. 7) is measured with a surface potential sensor 7 to thereby evaluate transfer memory. A larger decrease in surface potential xcex942 means a larger tendency to produce transfer memory.
Transfer memory is apt to be produced in reversal development using a transfer charger whose polarity is opposite to the polarity of a photoconductor. Therefore, generally, the capability to be charged drops more in reversal development in which both photo fatigue and transfer memory affect the capability than in normal development in which only photo fatigue affects it.
Further, when flash light impinges on a photoconductor that is under transfer charging by a transfer charger, the photoconductor is charged to have a polarity opposite to the polarity of the transfer charger at the same time that the flash light causes a decrease in surface potential of the photoconductor. Thus, the capability to be charged drops more. Further, the more the photoconductor is deteriorated due to repeated printing, the more the capability to be charged drops due to photo fatigue and transfer memory. FIG. 8 shows how the surface potential of the deteriorated photoconductor varies after each step.
As shown in FIG. 8, the capability of the photoconductor to be charged drops at its portions that have received flash light. Portions that have a lower surface potential after main charging are produced in accordance with the flash light being emitted periodically. If a decrease in surface potential xcex94V is large, it may cause stains on a white ground in reversal development and decrease in concentration in normal development.
A Various kinds of photoconductors such as amorphous silicon, selenium, cadmium sulfide and organic photoconductors show such drop in capability to be charged. Especially in the case of a positively-charged single-layer type organic photoconductor, electrons are apt to remain and drop in capability to be charged, therefore, decrease in surface potential xcex94V is particularly large, as shown in Unexamined Japanese Patent Publication (KOKAI) No. Hei 7-234618.
To deal with this problem, it is possible to make a paper carrying path on the flash lamp side sharply bent relative to a paper carrying path on the photoconductor side to thereby reduce the amount of flash light impinging on a photoconductor to thereby reduce drop in capability to be charged. However, when the paper carrying path is bent, thick paper and sized paper may not be carried well. Further, a toner image not fixed yet may touch a carrying guide and the like and cause deterioration in printing.
It is also possible to reduce the output of a flash lamp to thereby reduce the amount of flash light impinging on a photoconductor. However, this makes a toner image fixed to a recording medium worse.
The present invention has been made in view of the above problems. The object of the present invention is to provide an image forming method and image forming apparatus in which a recording medium is carried well and the possibility of producing stains on a white ground is low even if flash light impinges on an image carrier.
In order to attain the above object, the present invention provides an image forming method in which a toner image formed on an image carrier through steps of discharging, main charging, exposure and development is transferred to a recording medium and then fixed as an image using flash light, wherein after the toner image is transferred, the image carrier is subjected, prior to discharging, to secondary charging that gives the same polarity as the main charging gives and a surface potential larger in absolute value than the main charging gives.
Desirably, the recording medium is continuous paper.
Desirably, a carrying path along which the recording medium is carried while the toner image is transferred and then fixed is substantially a straight line.
Desirably, the development through which the toner image is formed is reversal development.
Desirably, the image carrier is an organic photoconductor.
Desirably, the flash light is emitted from a plurality of light sources simultaneously.
Further, in order to attain the above object, the present invention provides an image forming apparatus comprising at least an image carrier, main charging means, exposure means, development means, transfer means for transferring an image to a recording medium, discharging means, fixing means using a flash lamp, carrying means for carrying the recording medium from transfer position to fixing position, and secondary charging means for secondarily charging the image carrier to have the same polarity as a polarity given by the main charging means and a surface potential larger in absolute value than a surface potential given by the main charging means, the secondary charging means being arranged to act on the image carrier after the transfer means acts on the image carrier and before the discharging means acts on the image carrier.
Desirably, the recording medium is continuous paper.
Desirably, the carrying means carries the recording medium along a carrying path that is substantially a straight line.
Desirably, the development means is means for performing reversal development.
Desirably, the image carrier is an organic photoconductor.
Desirably, the fixing means comprises a plurality of flash lamps adapted to emit light simultaneously.
It is to be noted that in the specification, xe2x80x9clarger surface potentialxe2x80x9d means surface potential larger in absolute value. Further, in the specification, the absolute value of surface potential means the maximum in absolute value of surface potential that varies during printing.